The present invention relates to an apparatus and a method for bad frame detection in a digital communication system.
Methods for coding/decoding of speech signals in a radio communication system are well known and even standardized (for example IS-54 in the U.S. and GSM in Europe). Furthermore, methods for improving encoding/decoding of background sounds, primarily intended for digital cellular telephone systems, have been described in Swedish Patent Application 93 00290-5. Both these types of methods are primarily designed to handle a situation where the connection between the speech encoder and the speech decoder is close to ideal, in the sense that only a small amount of bit or transmission errors remain after channel decoding. However, since the connection is a radio channel the received signal may contain some bit or transmission errors. In such cases it may be necessary to modify the methods mentioned above.
Thus, it is essential to be able to reliably detect bad received frames in order to perform these modified methods. Usually a cyclic redundancy check (CRC) is used at the receiver as a quality measure. If the CRC fails this is an indication that the received frame may be bad. Another quality measure is so called soft information from the detector. This soft information essentially indicates the probability of the received speech frame (or parts of it) to be correct. A problem with the first type of measure (CRC) is that a speech frame is interleaved over a number of time slots. Since the CRC requires the bits from all these time slots before the check can be made, this implies a delay until the decision can be made and error concealment measures can be taken. A problem with soft information is that it is difficult to set thresholds correctly. If the thresholds are set too low a high false alarm rate will occur (acceptable frames treated as bad frames).
An object of the present invention is an apparatus and a method in which bad frames are more reliably detected to enable bad frame concealment in accordance with the above mentioned modified methods.